1. Field of the Invention
The present invention relates to a wireless communication apparatus for performing media access control on the basis of a carrier sense state.
2. Description of the Related Art
In recent years, wireless LANs (local area network) have become widespread in offices, homes, and as hot spot services in public places. In the mainstream as wireless LAN standards are those such as IEEE802.11a using the 5 GHz band and IEEE802.11b/g using the 2.4 GHz band. However, IEEE802.11e, in which QoS (quality of service) function is extended to MAC (medium access control) layer on IEEE802.11a/b/g, is also established as a standard. Further, at present, the standardization activity of IEEE802.11n extending both the physical layer and the MAC layer with a goal of achieving effective throughput equal to or larger than 100 Mbps is also advanced.
As for IEEE802.11n, as one approach to realization of speed-enhancement of the transmission rate, a method of extending the communication band is proposed. Conventionally, the bandwidth of one channel used in the IEEE802.11 wireless LAN is 20 MHz. In the method, two channels of this bandwidth are simultaneously used, thereby to realize 40 MHz band communication having a twofold bandwidth. It also becomes necessary concomitantly to perform media access control while targeting different channels coexisting in the same frequency at the time of extending the communication band. In IEEE802.11n, considered are both a media access control system, in which a centralized control station reserves a plurality of frequency channels one by one in sequence as described in Jpn. Pat. Appln. KOKAI Publication No. 2004-242893, and a system in which each terminal as described in EWC MAC Specification Version V1.0 Sep. 12, 2005, Internet <URL: http://www.enhancedwirelessconsortium.org/> performs CSMA/CA similar to IEE802.11 in an autonomous and distributed manner, thereby making media access. The former, i.e., the media access control system in which a centralized control station reserves a plurality of frequency channels one by one in sequence is called a phased coexistence operation (PCO) mode, and is a system in which a period of narrowband communication (20 MHz) using a single channel and a period of broadband communication (40 MHz) using a plurality of channels are time-shared. On the other hand, the latter, i.e., the system in which each terminal performs CSMA/CA similar to IEE802.11 in an autonomous and distributed manner, thereby making media access, is called a 20/40 MHz mode, in which switching of communication in units of one frame between a narrowband (20 MHz) frame using a single channel and a broadband (40 MHz) frame using a plurality of channels is permitted.
As described above, the PCO mode, 20/40 MHz mode, and 20 MHz only mode in which only narrowband (20 MHz) communication using a single channel as in conventional IEEE802.11 is performed exist as operational modes of BSS in IEEE802.11n. The BSS operational mode is selected by the centralized control station and this fact is notified to terminals in the BSS. In this case, there is a technical problem that if the centralized control station does not appropriately select the BSS operational mode, lowering of the throughput of the BSS is brought about.
For example, when a 40 MHz terminal is present in the BSS, if the 20 MHz only mode is selected as the BSS operational mode, the terminal can perform only communication using the 20 MHz band though the terminal has the 40 MHz communication capability, and low throughput is caused. On the other hand, when only a 20 MHz terminal is present in the BSS, if the PCO mode is selected as the BSS operational mode, a 40 MHz period is set though a terminal for transmitting in the 40 MHz band is not present, and switching between 20 MHz and 40 MHz is also caused, thereby wasting time and deteriorating the throughput.